Elegant handling of prompt overflow errors and warnings in Streamlit

[eddotman]

Following up on #2, we should also handle the specific case of the demo Streamlit.

In general, it should work fine to raise errors / warnings as appropriate if the prompt becomes too large. The Streamlit demo could elegantly handle these errors 1) for better user experience and 2) to show an example to users of how we might handle these errors in general.